Method and system for coordinated engine and transmission control during traction control intervention

ABSTRACT

A coordinated engine and transmission traction control method is disclosed for reducing driven wheel speed transients related to transmission shifting during a traction control event. The method includes sensing a driven wheel speed, and estimating a driving surface coefficient of friction based on the sensed wheel speed. The method also includes prolonging a transmission upshift by reducing a rate of change of a commanded speed ratio in proportion to the estimated driving surface coefficient of friction, and decreasing engine torque during the transmission upshift. A system including a wheel speed sensor and control logic is also disclosed for performing the method.

This is a continuation of application Ser. No. 08/784,216, filed on Jan.16, 1997, now U.S. Pat. No. 6,009,967.

TECHNICAL FIELD

This invention relates to a coordinated engine and transmission tractioncontrol method and system for reducing driven wheel speed transientsrelated to transmission shifting during a traction control event.

BACKGROUND ART

It is increasingly common for automotive vehicles to be equipped withtraction control systems. Such systems are designed to control the speedof a driven wheel to a desired level in order to improve vehiclestability, particularly on driving surfaces having low coefficients offriction.

More specifically, an imbalance between engine torque and road torqueacting on a vehicle wheel can cause the wheel speed to exceed thedesired level, which is typically referred to as wheel spin. To controlsuch wheel spin, traction control systems generally employ engine orbrake management to control the engine torque acting on the wheel.

In automatic transmission vehicles, while operating in a tractioncontrol mode, shifting of the automatic transmission powertrain can alsocontribute to such torque imbalances. Indeed, such shifting can causesudden and significant disturbances in driven wheel speeds. In turn,such shift-created wheel speed transients can momentarily reduce vehiclestability.

As a result, there is a need for an improved traction control method andsystem that reduces driven wheel speed transients related totransmission shifting during a traction control event. Such a method andsystem would reduce such transients using coordinated engine andtransmission control.

SUMMARY OF THE INVENTION

Accordingly, it is the principle object of the present invention toprovide an improved traction control method and system using coordinatedengine and transmission control to reduce driven wheel speed transientsrelated to transmission shifting during a traction control event.

According to the present invention, then, a coordinated engine andtransmission traction control method is provided for reducing drivenwheel speed transients related to transmission shifting during atraction control event. The method comprises sensing a driven wheelspeed, and estimating a driving surface coefficient of friction based onthe sensed wheel speed. The method further comprises prolonging atransmission upshift by reducing a rate of change of a commanded speedratio in proportion to the estimated driving surface coefficient offriction, and decreasing engine torque during the transmission upshift.

A coordinated engine and transmission traction control system forreducing driven wheel speed transients related to transmission shiftingduring a traction control event is also provided. The system comprises awheel speed sensor for sensing a driven wheel speed. The system furthercomprises control logic operative to estimate a driving surfacecoefficient of friction based on the sensed wheel speed, prolong atransmission upshift by reducing a rate of change of a commanded speedratio in proportion to the estimated driving surface coefficient offriction, and decrease engine torque during the transmission upshift.

These and other objects, features and advantages will be readilyapparent upon consideration of the following detailed description of theinvention in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a is a graph of driven and non-driven vehicle wheel speeds overtime on a driving surface having a low coefficient of friction accordingto prior art traction control methods and systems;

FIGS. 1b and 1c are graphs of driven and non-driven vehicle wheel speedsover time in relation to vehicle gear shifting according to prior arttraction control methods and systems;

FIG. 2 is a graph illustrating automatic transmission gear shifting inrelation to throttle angle position and transmission output speed;

FIG. 3 is a graph comparing speed ratios over time according to priorart traction control methods and systems and the traction control methodand system of the present invention;

FIG. 4 is a simplified flowchart of the method of the present invention;and

FIG. 5 is a simplified block diagram of the system of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

In general, on slippery driving surfaces where traction controlintervention is required, the method and system of the present inventionemploy an altered transmission control and a coordinated transmissionand engine control. In so doing, the traction control method and systemof the present invention significantly reduce shift-induced wheel speeddisturbances on slippery surfaces, thereby improving vehicle stability,especially in turns.

Referring to FIGS. 1-5, the preferred embodiment of the presentinvention will now be described in greater detail. In that regard, FIG.1a illustrates driven and non-driven vehicle wheel speeds (in RPMs) overtime on a driving surface having a low coefficient of friction (μ)according to prior art traction control methods and systems.

More specifically, a driven (front) wheel speed (10), a non-driven(rear) wheel speed (12), and a desired driven wheel speed (14) are shownfor a vehicle equipped with an engine-only (electronic throttle andspark) traction control system during a straight ahead winter test fromzero to 200 yards on polished ice.

As previously discussed, when operating in a traction control mode, anautomatic transmission powertrain can act as a significant and suddendisturbance to a driven wheel speed due to shift-created transients. Inthat regard, as seen in FIG. 1, between approximately three to 16seconds into the test, the vehicle is in first gear and the driven wheelspeed (10) follows the desired driven wheel speed (14) in aslip-controlled traction control event.

However, at approximately 16 to 17 seconds into the test, the vehiclegoes through the first to second gear upshift while in traction controlmode. Due to the inertia phase loading of the first to second gearpower-on upshift, the increased wheel torque causes a sudden spin ofdriven wheel (10) in the amount of 100 rpm or more. Such a suddenincrease in driven wheel (10) in turn leads to NVH Noise VibrationHarshness transients and momentary loss of directional control.

Referring now to FIGS. 1b and 1c, driven and non-driven vehicle wheelspeeds are illustrated over time in relation to vehicle gear shiftingaccording to prior art traction control methods and systems. As seentherein, when operating on slippery surfaces such as ice or snow,traction control equipped vehicles will exhibit a first spincharacteristics where the driven wheel speed average (10) increasessignificantly faster than the non-driven wheel speed average (12).

At present, transmission shift schedules are often established as curveswhich are a function of throttle angle position versus vehicle speed.Significantly, vehicle speed is measured at the driven wheels. As aresult, it is often possible that a relatively high driven wheel speedaverage, such as that which occurs during wheel spin, may induce atransmission upshift. This is depicted in FIGS. 1b and 1c at time t₁. Itis also possible that once the initial spin has been contained, atransmission downshift to the original gear may occur as depicted inFIGS. 1b and 1c at time t₂.

To avoid such unnecessary shifting, which can be detrimental todrivetrain/transmission durability as well as vehiclestability/performance on slippery surfaces such as ice or snow, themethod and system of the present invention use true vehicle speed forshift scheduling rather than the driven wheel speed average ortransmission output speed, which is described in greater detail below.The true vehicle speed may be provided as an average of the non-drivenwheel speeds in the case of two-wheel drive vehicles equipped with atleast two or three channel anti-lock brake systems (ABS). In that case,the ABS wheel speed sensors can be used to accomplish this task.

In vehicles without ABS, true vehicle speed can besynthesized/approximated using the transmission output speed sensor(OSS). The OSS signal should be heavily filtered with a low-pass filterhaving a bandwidth of 0.5-1.0 Hz or lower. This will substantiallyreduce the first spin magnitude and thus prevent the unnecessary upshifttrigger. This could be augmented by limiting the slope of the wheelspeed traces to below the maximum slope physically possible prior tolow-pass filtering.

An alternative methodology for determining true vehicle speed invehicles without ABS could be based on wheel torque estimation asdisclosed in U.S. Pat. No. 5,452,207, which is hereby incorporated byreference. A vehicle model based on such methodology would beinitialized with a vehicle speed, road grade, and air resistance justbefore the onset of wheel spin.

Additionally, the above approaches could be combined via fuzzy logic toresult in an effective true vehicle speed estimate for use in thealtered shift scheduling of the method and system of the presentinvention. Still further, in four-wheel drive or ABS vehicles withacceleration-based speed signal reconstruction, the same accelerationsensor could be used for true vehicle speed estimation.

To prevent the wheel speed disturbance effects described above, themethod and system of the present invention employ an altered automatictransmission control strategy during traction control mode. In thatregard, the method and system of the present invention are designed foruse with an electronically-controlled transmission well known in the arthaving appropriate actuators.

The first alteration to the automatic transmission control strategypertains to the shift scheduling during traction control mode. Here, themethod and system of the present invention use the effective throttleangle in the case of series-throttle actuator. More particularly,effective throttle angle may be calculated based on correspondingeffective flows, as described in U.S. Pat. No. 5,520,146 which is herebyincorporated by reference.

Moreover, upshift during the first driven wheel spin can causeespecially problematic wheel speed transients when operating in atraction control mode. As a result, the method and system of the presentinvention also delay a requested upshift until after driven wheel spinis first exhibited during traction control. In the case where no upshifthas yet been requested, the method and system of the present inventioncommand or introduce an upshift after driven wheel spin is firstexhibited during traction control.

More specifically, referring now to FIG. 2, automatic transmission gearshifting is illustrated in relation to throttle angle position (TAP) andtransmission output speed (N_(out)) (in RPMs) . As seen therein, shiftcurve (20) shows the TAP (22) or N_(out) (24) at which a shift will becommanded between a lower gear (A) and a higher gear (B) for a givenN_(out) (24) or TAP (22), respectively. It should be noted that shiftcurve (20) is exemplary only, and may differ from vehicle to vehicle. Itshould also be noted that the above discussion concerning shiftscheduling as a function of throttle angle position versus vehicle speedwould result in a similar curve to that depicted in FIG. 2, wheretransmission output speed N_(out) would be replaced by vehicle speed asrepresented by the driven wheel speed average.

Referring now to FIGS. 1a and 2, during traction control intervention,if the TAP and N_(out) are such that a gear shift is commanded at thetime driven wheel spin is first exhibited, transmission control isaltered to delay such a shift to a later time when such wheel spin hasbeen contained. Alternatively, if the TAP and N_(out) are such that agear shift has not yet been commanded at the time driven wheel spin isfirst exhibited, transmission control is altered to introduce such ashift shortly after that time, again when such wheel spin has beencontained. In either fashion, the method and system of the presentinvention significantly reduce shift-induced wheel speed disturbances onslippery surfaces, thereby improving vehicle stability.

Moreover, shift scheduling may also be changed if an upshift is stillencountered during the first exhibited wheel spin. In that case, such anupshift can be prevented by changing the shift (new gear) command to beequal to the present gear when wheel spin is detected

To still further prevent the large upshift-created transients depictedin FIG. 1a, the method and system of the present invention alsocoordinate transmission and engine control. More specifically, once in atraction control operating mode, an upshift is prolonged through moregradual oncoming clutch (or band) element pressure application, whichshould be appropriately reduced.

In the case of closed-loop speed ratio control, a prolonged upshift canbe achieved by commanding a less steep speed ratio (SR), which isdefined as in automatic transmission vehicles as transmission outputspeed (N_(out)) (in RPMs) divided into turbine speed (N_(t)) (in RPMs) .In that regard, FIG. 3 illustrates SR over time for power-on shiftingaccording to prior art traction control methods and systems and thetraction control method and system of the present invention. As seentherein, the traction control method and system of the present inventioncommand a SR (26) having a reduced rate of change over time as comparedto the commanded rate of change of SR (28) of prior art methods andsystems. In such a fashion, the method and system of the presentinvention achieve a prolonged upshift, especially the inertial phase ofthe upshift between time t₃ and time t₄. It should be noted thatprolongation of the transmission upshift during the inertial phaseshould take into account clutch thermal effects to prevent clutchdamage.

The reduction in the slope of the curve of speed ratio (26) commanded bythe method and system of the present invention is a function of thecoefficient of friction (μ) of the driving surface, which may beestimated in any known fashion. In that regard, the slope of SR curve(26) is reduced as μ decreases.

More particularly, the limiting torque at the wheel (t_(lim)) which thedriving surface can sustain is a function of the estimated μ of thedriving surface. Such limiting torque must be greater than the actualtorque at the wheel (t_(w)) This relationship is represented by thefollowing constraint equation:

    t.sub.w <t.sub.lim (μ.sub.est)                          (1)

In that regard, t_(w) may be represented as follows:

    t.sub.w =SR.sub.ng *f.sub.tc (SR.sub.tc, (t.sub.e +I.sub.e a.sub.e))*FDR/2*e.sub.eff                                 (2)

where SR_(ng) is the speed ratio of the next gear; f_(tc) is a torqueconversion function based on the speed ratio of the torque convertorSR_(tc), engine torque t_(e), engine inertia I_(e), and engineacceleration a_(e) ; FDR is the final drive ratio; and e_(eff)represents transmission efficiency and includes those factors loweringtorque delivered at the wheel. In that regard, e_(eff) is less than one(approximately 0.9).

As is well known in the art, the slope of the curve of SR (26) isrelated to engine acceleration (a_(e)) through a torque conversionfunction. Thus, according to the method and system of the presentinvention, the rate of change of SR (26) over time is reduced accordingto the following constraint equation:

    SR.sub.ng *f.sub.tc (SR.sub.tc, (t.sub.e +I.sub.e a.sub.e))*FDR/2*e.sub.eff <t.sub.lim (μ.sub.est)                                 (3)

In addition to the above transmission control, engine torque can also besimultaneously controlled via throttle, spark and fuel adjustments (orany combination thereof) in any well known fashion. For example, with anadvanced indication of a planned upshift, the method and system of thepresent invention may use such preview control to lower (i.e., startclosing) the throttle to timely reduce the engine torque during theinertia phase so that the resulting wheel torque is not significantlydisturbed. Alternatively, the engine spark may be timely retarded and/orthe engine fuel supply reduced for the same purpose. In that sameregard, engine torque may also be controlled using known methods andsystems directed to synthetic throttle and/or wheel torque estimation,such as that disclosed in the previously mentioned U.S. Pat. No.5,452,207. Indeed, wheel torque better reflects the actual road load,especially during initial wheel spin.

Such control, in turn, can also lead to reduced requirements for theshift duration increase described above. In that regard, control of theengine via throttle, spark and/or fuel adjustments is undertaken toreduce engine speed (N_(e)), which reduces engine torque (t_(e)).Referring to equation (3) above, a reduction in engine torque (t_(e))means that engine acceleration (a_(e)) may be higher. As previouslystated, engine acceleration (a_(e)) is related to the slope of SR (26)through a torque conversion function. In that regard, if engineacceleration (a_(e)) may be higher, the slope of SR (26) may be steeper,thereby reducing the requirements for increasing the duration of theshift.

Referring next to FIG. 4, a simplified flowchart of the method of thepresent invention is shown. As seen therein, a determination is firstmade (30) as to whether the vehicle is operating in a traction controlmode. If not, the method of the present invention is not performed (32).

However, if the vehicle is operating in a traction control mode,"when-to-shift" alterations are made (34) to the transmission controlstrategy. More specifically, the shift schedule of the transmission isfirst altered. As previously described, such an alteration of the shiftschedule may include delaying a requested upshift until after drivenwheel spin is first exhibited during traction control, or commanding anupshift after driven wheel spin is first exhibited during tractioncontrol. In either case, upshift is delayed or introduced after suchwheel spin has been contained. In that regard, as also previouslydescribed, such alteration of the shift schedule is based on a number ofinputs such as transmission output speed (N_(out)) (36), throttle angleposition (TAP) (38), and wheel speed (not specifically shown).

In addition, "how-to-shift" alterations are also made (40) to thetransmission control strategy. More specifically, as previouslydescribed, such alterations include prolonging an upshift based on aninput of the estimate coefficient of friction of the driving surface (μ)(42). In that regard, as also previously described, such prolonging ofan upshift is also based on a number of other inputs includingtransmission output speed (N_(out)) (44) and turbine speed (N_(t)) (46)(for use in determining speed ratio (SR) as well as engine acceleration(a_(e))).

As also previously described, such "how-to-shift" alterationsrations mayalso include decreasing engine torque delivered to the wheel during anupshift by retarding engine spark, reducing engine throttle and/orreducing engine fuel supply in any known fashion. In that regard, asdescribed above, such engine torque control is based on a number ofinputs including engine speed (N_(e)) (48) (in RPMs).

Referring finally to FIG. 5, a simplified block diagram of the system ofthe present invention is shown. As seen therein, the system is designedfor use in a vehicle preferably having a pair of front driven wheels(50, 52), a pair of rear non-driven wheels (54, 56), and an engine (58)for driving front wheels (50, 52) via transmission (60).

In the preferred embodiment, the traction control system of the presentinvention comprises wheel speed sensors (62, 64, 66, 68) associated witheach of the driven and non-driven wheel (50, 52, 54, 56). The systemfurther comprises a controller (70) operatively connected to wheel speedsensors (62, 64, 66, 68), engine (58) and transmission (60). In thatregard, controller (70) receives wheel speed input signals from wheelspeed sensors (62, 64, 66, 68) and transmits control signals to engine(58) and transmission (60).

In the preferred embodiment, controller (70) is a conventionalmicroprocessor appropriately programmed to perform various aspects ofthe method of the present invention. As is readily apparent to those ofordinary skill in the art, however, any equivalent thereof may also beused. It should also be noted that the aspects of the method of thepresent invention may be undertaken in any sequence and/orsimultaneously.

The appropriately programmed controller (70) (or its equivalent) serveas control logic operative to estimate a driving surface coefficient offriction based on the sensed wheel speed, prolong a transmission upshiftby reducing a rate of change of a commanded speed ratio in proportion tothe estimated driving surface coefficient of friction, and decreaseengine torque during the transmission upshift.

In that regard, as previously described, such control logic is operativeto reduce the rate of change of the speed ratio according to thefollowing constraint equation:

    SR.sub.ng *f.sub.tc (SR.sub.tc, (t.sub.e +I.sub.e a.sub.e))*FDR/2*e.sub.eff <t.sub.lim (μ.sub.est)                                 (3)

where SR_(ng) is the speed ratio of the next gear; f_(tc) is a torqueconversion function based on the speed ratio of the torque convertorSR_(tc), engine torque t_(e), engine inertia I_(e), and engineacceleration a_(e) ; FDR is the final drive ratio; e_(eff) representstransmission efficiency and includes those factors lowering torquedelivered at the wheel; and t_(lim) (μ_(est)) is the limiting torque ata driven wheel as a function of the estimated driving surfacecoefficient of friction. Once again, e_(eff) is less than one(approximately 0.9).

Still further, to prolong a transmission upshift, such control logic isoperative to reduce a rate of application of fluid pressure to atransmission shifting element as previously described. Moreover, asdescribed above, to decrease engine torque, such control logic isoperative to retard an engine spark, reduce an engine throttle and/orreduce an engine fuel supply in any known fashion. Such control logic isstill further operative, as also previously described, to identify afirst driven wheel spin during the traction control event, and eitherdelay the transmission upshift to a time after the first driven wheelspin, or introduce the transmission upshift after the first driven wheelspin.

As is readily apparent to those of ordinary skill in the art, then, thepresent invention provides an improved traction control method andsystem that reduces driven wheel speed transients related totransmission shifting during a traction control event. Morespecifically, the method and system of the present invention reduce suchtransients using coordinated engine and transmission control.

It is to be understood that the present invention has been describedabove in an illustrative manner and that the terminology which has beenused is intended to be in the nature of words of description rather thanof limitation. As previously stated, many modifications an variations ofthe present invention are possible in light of the above teachings.Therefore, it is also to be understood that, within the scope of thefollowing claims, the invention may be practiced otherwise than asspecifically described herein.

What is claimed is:
 1. A coordinated engine and transmission tractioncontrol method for reducing driven wheel speed transients related totransmission shifting after a first occurrence of driven wheel spin, themethod comprising:sensing a driven wheel speed; estimating a drivingsurface coefficient of friction based on the sensed driven wheel speed;reducing a rate of change of a transmission speed ratio in proportion tothe estimated driving surface coefficient of friction to prolong atransmission upshift; and decreasing engine torque during thetransmission upshift.
 2. The method of claim 1 wherein the rate ofchange of the speed ratio is reduced according to the constraintequation:

    SR.sub.ng *f.sub.tc (SR.sub.tc, (t.sub.e +I.sub.e a.sub.e))*FDR/2*e.sub.eff <t.sub.lim (μ.sub.est)

where SR_(ng) is the speed ratio of the next gear; f_(tc) is a torqueconversion function based on the speed ratio of the torque convertorSR_(tc), engine torque t_(e), engine inertia I_(e), and engineacceleration a_(e) ; FDR is the final drive ratio; e_(eff) representstransmission efficiency and includes factors lowering torque at a wheel;and t_(lim) (μ_(est)) is the limiting torque at a driven wheel as afunction of the estimated driving surface coefficient of friction. 3.The method of claim 1 wherein reducing a rate of change of atransmission speed ratio includes reducing a rate of application offluid pressure to a transmission shifting element.
 4. The method ofclaim 1 wherein the transmission upshift is based on an estimated truevehicle speed.
 5. The method of claim 1 wherein decreasing engine torquecomprises retarding an engine spark.
 6. The method of claim 1 whereindecreasing engine torque comprises reducing an engine throttle.
 7. Themethod of claim 1 wherein decreasing engine torque comprises reducing anengine fuel supply.
 8. The method of claim 1 further comprising:delayingthe transmission upshift to a time after the first occurrence of drivenwheel spin has been reduced to a desired level.
 9. The method of claim 1further comprising:introducing the transmission upshift after the firstoccurrence of driven wheel spin has been reduced to a desired level. 10.A coordinated engine and transmission traction control system forreducing driven wheel speed transients related to transmission shiftingafter a first occurrence of driven wheel spin, the system comprising:awheel speed sensor for sensing a driven wheel speed; and control logicoperative to estimate a driving surface coefficient of friction based onthe sensed driven wheel speed, reduce a rate of change of a transmissionspeed ratio in propotion to the estimated driving surface coefficient offriction to prolong a transmission upshift, and decrease engine torqueduring the transmission upshift.
 11. The system of claim 10 wherein therate of change of the speed ratio is reduced according to the constraintequation:

    SR.sub.ng *f.sub.tc (SR.sub.tc, (t.sub.e +I.sub.e a.sub.e))*FDR/2*e.sub.eff <t.sub.lim (μ.sub.est)

where SR_(ng) is the speed ratio of the next gear; f_(tc) is a torqueconversion function based on the speed ratio of the torque convertorSR_(tc), engine torque t_(e), engine inertia I_(e), and engineacceleration a_(e) ; FDR is the final drive ratio; e_(eff) representstransmission efficiency and includes factors lowering torque at a wheel;and t_(lim) (μ_(est)) is the limiting torque at a driven wheel as afunction of the estimated driving surface coefficient of friction. 12.The system of claim 10 wherein, to reduce a rate of change of atransmission speed ratio, the control logic is operative to reduce arate of application of fluid pressure to a transmission shiftingelement.
 13. The system of claim 10 wherein the transmission upshift isbased on an estimated true vehicle speed.
 14. The system of claim 10further wherein the control logic is further operative to introduce thetransmission upshift after the first occurrence of driven wheel spin hasbeen reduced to a desired level.
 15. The system of claim 10 wherein, todecrease engine torque, the control logic is operative to retard anengine spark.
 16. The system of claim 10 wherein, to decrease enginetorque, the control logic is operative to reduce an engine throttle. 17.The system of claim 10 wherein, to decrease engine torque, the controllogic is operative to reduce an engine fuel supply.
 18. The system ofclaim 10 wherein the control logic is further operative to delay thetransmission upshift to a time after the first occurrence of drivenwheel spin has been reduced to a desired level.